Various types of chairs, particularly desk chairs, chairs to be used with work tables, drafting tables, and the like, have a combination seat-and-back tilting mechanism which permits, simultaneously, tilting back of the seat and of the back of the chair up to a predetermined fixed position; or to lock the chair in a desired predetermined e.g. tilted position. Chairs in which the seat and back tilt together are also referred to as synchronous work chairs or synchronous desk chairs, since adjustment of the inclination of the back of the chair causes simultaneously a shift of the inclination of the seat or seat plate.
Synchronized desk chairs usually have a spring, such as a locking-type spring element. The spring may be a mechanical spring, or may be a gas spring, a hydraulic arrangement, similar to a dash-pot, or the like. When the spring unit is not locked, the operator can tilt the seat and back of the chair, that is, change the inclination of the respective seat and back elements in accordance with desired posture or the work to be carried out. Any particular tilt can be locked in position by locking the spring when a specific tilt has been achieved by the user. The user may, of course, also release the lock, for example to obtain a more comfortable position, and then, again, lock the spring to maintain the particular inclination which was desired.
To lock and provide a tilting force or, rather, a counter force to the weight of the user, requires a lever mechanism which, customarily, is arranged beneath the seat, or in the back thereof. This lever arrangement interconnects the seat plate, the carrier frame for the back, and the spring unit, including its locking mechanism. The lever arrangement is usually so designed that the inclination of the back of the chair and of the seat have predetermined relationships with respect to each other, which are not necessarily linear, or strictly proportional. Such lever arrangements are costly to make, and comparatively large, and it is not an easy matter to fit the mechanism beneath the seat plate without extending the apparatus for the mechanism substantially below the seat plate. Use of a plurality of links or joints between the individual adjustment levers has an additional disadvantage: If the seat back is tilted frequently, the joints between the links or levers will become loose due to wear at bearings. The wear at different bearings usually becomes additive and, in the long run, the mechanism no longer will be stiff but, since chattering or loose bearings will be within the overall mechanism, it is no longer possible to lock the chair in a single predetermined position; rather, the inclination will oscillate about a medium, and shifting of weight of the user, or shifting of the user's position, causes wobble of the seat plate and the seat back, frequently accompanied by noise, which is highly undesirable.
A desk chair is described in Swiss Pat. No. 429,537, in which both the seat plate as well as the back support, or back frame, have link levers extending beneath the seat plate, secured to the seat carrier. The seat plate is tilted in dependence on the tilt of the back support by a double link lever which connects the seat plate with the seat carrier at a link point which is spaced from the link position of the arm of the back of the chair or its back support frame. A gas spring is used to counterbalance the weight of the user upon tilting of the seat plate and, in one structure, provides force tot he seat plate, to the back support carrier, and to the double link lever and the other linkages in connection therewith. The gas spring can be locked, so that a predetermined inclination of the seat plate and the back support frame can be obtained. The gas spring is positioned practically perpendicularly with respect to the seat plate. The overall construction of the unit is comparatively simple; yet, a relatively large movement between the back of the chair and the back of the user results when the seat is tilted or shifted. As a consequence, a user wearing a shirt or a blouse will, by frictional engagement, have the shirt or blouse pulled out from trousers or skirts. This relative movement has been termed a "shirt take-off" effect. The main reason for the relatively large movement between the back of the chair and the back of the user is the position of the pivot axis of the support element for the back of the chair, which remained stationary, whereas the position of the hip joint of the user, which forms the pivot axis for the body of the user, together with the seat plate, moves upwardly and downwardly. Gas springs have a relatively flat spring characteristic. This introduces an additional disadvantage in the particular construction: If the user moves backwardly against the back support chair, compensation of the of the weight of the user by the gas spring itself is not effectively obtained.
It has already been proposed to overcome the problem of pullout of a shirt or blouse from trousers, slacks or skirts by connecting the back support of the chair to a cable which, in turn, is connected to a slider located on the chair base or beneath the chair seat, so that movement of the back towards the top or towards the bottom can be effected with movement of the seat. This is intended to eliminate relative movement between the back of the user and the back support of the chair. Such a structure is comparatively expensive and subject to malfunction, for example jamming of the cable, guide pulleys or the like.